Idleness Is A Fuel's Desire

We moan about the rising cost of gasoline, but we keep putting it in our cars as, without it, they're expensive garage ornaments. But until you build a project, or have to replace parts, the fuel system goes largely ignored. As with most things in life, times change, and products improve. Not to mention the big shift towards fuel injection means components that worked with carbureted engines will no longer work. With these thoughts in mind, let's take a look at how the precious lifeblood gets from your gas cap to the combustion chamber.

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Broken down into its component parts, a fuel system comprises the tank, fuel lines and fittings, either a mechanical or electric pump, filters, and a regulator. If you haven't plumbed a fuel system since the days of running a hardline from the tank to a mechanical pump at the engine, with a filter between that and the carburetor, you'll be surprised how complicated things got. But then, one look at the fuel system on your daily driver will confirm that! Fuel injection has been around since the Fifties on production cars, and has been used exclusively on cars sold in the United States since 1990. While many hot rodders still prefer to run a carburetor, an increasing number are embracing fuel injection technology. While we won't discuss the ins and outs of fuel injection here, its basic requirement is that it be fed gasoline under pressure in order to operate, at much higher pressures than a carbureted fuel system requires, and indeed at much higher pressures than a mechanical pump can generate, which is why you'll find electric fuel pumps on fuel injection systems. Edelbrock recommends a pressure of 5.5 psi for its Performer carburetors, compared to 14 psi for a stock '89 GM 305ci throttle body unit and around 45 psi for tuned port injection systems. Quite a difference!

Of course you can run an electric pump on a carbureted engine. Holley's Red pump pushes 7 psi, and a Carter even less; coupled with an in-line regulator it's possible to control the pressure at the carburetor. But why would you want to replace a perfectly good mechanical pump anyway? Sure they're low tech, quiet and reliable and don't require any wiring, plus they shut off when the engine dies, but they're also located in an area of extreme heat, which can cause vapor lock, and require the engine to be cranked over excessively if it hasn't been fired for a while, in order to pump gas to the carb, as they first have to create a vacuum in order to pull the gas all the way from the tank. This period of cranking the engine over before firing can promote excessive bearing wear. If you pump the gas pedal, especially on a hot engine that may be having trouble starting, depending on the carburetor it may set the choke mechanism, which can lead to bore wash, where fuel will wash the cylinder walls, again promoting undue wear. On the plus side, they're easy to plumb, and simple to find a replacement should you have to do so while on the road. No small consideration with the mileage we're putting on our hot rods these days, and one of the reasons SO-CAL Speed Shop likes to use factory style mechanical pumps on their own and customer cars. Indeed, you may not even be aware that Holley sells mechanical pumps alongside its range of electric versions.

A problem that we're hearing more and more these days with new fuel compositions, and fuels such as E85, is issues with pumps and even flexible lines. The diaphragm in some pumps, especially older ones, may break down, owing to their intolerance of these new fuels. I have had firsthand experience of this, with the diaphragm in a cheap electric pump turning to a sticky gloop after sitting for a few months with fuel in it. This is one reason why the OEMs have switched to plastic fuel lines rather than rubber.

If you've been messing with old cars for a while, your experience of electric pumps may be limited to something like the Holley Red or Blue pumps, but the aftermarket has moved along with the OEM fuel systems, and in-line and even in-tank pumps are now commonplace. There are several advantages to in-tank pumps; the fuel helps keep the submersed pump cool and the risk of fire is reduced, as liquid fuel won't explode. Any pump will work better the closer it is to the fuel tank, and in-tank pumps totally eliminate the need to create a vacuum and "pull" fuel up to the pump, ensuring a steady supply. This is especially important with an injected engine, which will stutter or die at even the briefest of lapses in fuel supply.

Of course to maintain that steady supply, the pump pickup has to be constantly immersed in fuel. According to John Pazik at Tanks, Inc., "If the tank is not equipped with a reservoir tray to keep the pump inlet immersed in fuel, the pump will suck air when turning corners or accelerating and the engine will stumble." Many of the company's die-stamped tanks are manufactured with an internal reservoir tray, but they also offer a pump kit which has the tray, and can be installed in your existing tank simply by cutting a 41/2-inch hole and a series of mounting holes, with no welding required. Rock Valley also manufactures tanks with an internal pump, though theirs is mounted in a canister type reservoir rather than a tray, both doing the same job. The company makes a number of stainless steel tanks for early cars, as well as dedicated in-tank pump and sender kits for specific fuel tanks, though welding is required to fit these, either by the customer, or by shipping the tank to Rock Valley to be modified.

An advantage to using a new fuel tank like those above is that it'll be free of debris and will already be equipped with an outlet of sufficient diameter to deliver the quantity of fuel required. This is something to think about if you plan to replace your entire fuel system but retain the stock tank. While the OEM-style in-tank pump has advantages, in-line pumps are also available, whether you plan on running EFI or carbs, or just don't want to modify your tank. If this is the route you take, according to Jesse Powell of Aeromotive, "An external fuel pump, either electric or mechanical, would ideally be placed as low and as close to the tank as is possible. You want to minimize the vacuum the pump has to make to get fuel from the tank. This avoids premature fuel vaporization and extends the fuel pump's duty cycle and ability to supply liquid fuel as fuel temperature elevates on hot days. Keep in mind that any liquid's boiling temperature drops as you pull a vacuum on it."

We mentioned constant fuel supply is important to an injected engine, and this is because, unlike a carburetor, there's no "reserve" fuel, such as a carb has, in the float bowl or bowls. Injection systems have a return line that sends unused fuel back to the tank, and this should be at least 5/16-inch in diameter if the supply line is 3/8-inch. The return line should also feed back to the bottom of the fuel tank. The use of a return line reduces the chances of fuel boiling and causing vapor lock problems, because the fuel is never kept close to a hot engine for very long.

While carbureted fuel systems are usually of a "deadhead" design, with no return line, but merely a regulator determining the rate of fuel flow to the carburetor, there are advantages to switching to a return style system. Jesse Powell again; "There is much potential for improvement in flow and pressure control available by incorporating bypass regulators and return lines in carbureted applications. This is particularly true when fuel pumps that do not have their own bypass capability are matched to regulators that do. In the big picture, there's not a single fuel system performance standard that doesn't improve when converted from deadhead to return style. For example, by running a bypass regulator, the pump can be set to run at lower pressure, thereby producing more flow, drawing less current, running quieter, even lasting longer. Return style systems respond faster to fuel demand and do a better job of keeping the float bowl full of fuel."

Of course, that fuel, whether sitting in a float bowl or pumping through an injector, has to be as clean as possible and free from debris, that is unless you want your jets or injectors to become blocked in short order. This is what filters are for. In the past we've used those cheapo white or clear plastic filters in our fuel systems; you know, the ones that cost a couple of bucks each at most, though even they've done their job and prevented dirt and rust from old fuel tanks from reaching the carburetor, but if you're investing money in a decent fuel system you'll want to step it up a few notches. Of course, if you're using fuel injection, you won't be able to use cheap filters as they won't take the pressure. This is a good time to mention that you should ensure all fuel lines are fuel injection type and not general fuel hose, which won't stand the pressure of an injected system. You really don't want a flexible hose coming away or splitting while gas is pumping through it under pressure!

So what should you look for in a filter? For starters it needs to minimize pressure drop while still being able to filter the fuel, but most importantly a filter absolutely must flow whatever the pump flows, making it "invisible" to the pump, with no restriction. This is achieved through a combination of its porosity, measured in microns, and its surface area. While any filter with a micron rating lower than 100 is too restrictive for the suction side of a pump, on the pressure side (between the pump and the engine) at least a 40 micron rating should be used. In fact Aeromotive suggest going down as low as 10 microns here. Jesse Powell told us "We have very specific filters that we match to different fuel pumps. Some of our larger fuel pumps require a larger filter with more surface area than others. We know this because we have spent an enormous amount of time testing our fuel pumps. We should know that each specific fuel pump requires a minimum amount of filtration media to ensure there is no restriction or pressure drop. We do this because if these fuel pumps operate in their ideal scenarios, the durability and performance is dramatically increased."

Maintaining filters is important to your fuel system's life too. Aeromotive recommends cleaning or replacing the element after the first 5-10 run hours, stating that the dirtiest a system will ever be is before it is installed. Paper elements should be replaced, but stainless steel ones can be re-used after cleaning, and annual filter servicing isn't a bad idea.

One area of the fuel system we haven't mentioned yet is the hard lines and fittings. Try to avoid sharp bends when fabricating the lines, in order to attain maximum flow, and avoid running it near exhaust systems. 3/8-inch (or #6 if you're familiar with that designation) tubing is sufficient for all but very high-performance applications, where you may need 1/2-inch (#8). Obviously, any fittings should be the same internal diameter as the tubing, so as not to cause obstructions to the fuel flow. The same advice regarding sharp bends applies here, especially with 90-degree fittings! While we're talking fuel flow, how do you calculate the fuel flow rate for your engine, and which pump should you select? According to Holley, an engine requires 1/2 lb of fuel for every horsepower at wide-open throttle. With a gallon of gas weighing 6 lb, this translates to a 350hp engine requiring 175 lb, or 29 gallons per hour (gph). This will tell you the maximum gph-rated pump you should use.

Hopefully we've given you an insight into modern fuel systems, why you should use fuel injection spec flexible lines, what to look for in a fuel pump, regulator and filters, and what's needed should you want to switch from a carburetor to fuel injection. All the companies whose products are shown here have websites that can explain specific applications, so do a little research and be safe. Ruptured fuel lines or leaks don't mix with hot engines, exhausts, or sparks.